Speed of Comparison operators

In languages such as... well anything, both operators for < and <= (and their opposites) exist. Which would be faster, and how are they interpreted?

if (x <= y) { blah; }

or

if 开发者_运维问答(x < y + 1) { blah; }

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Assuming no compiler optimizations (big assumption), the first will be faster, as <= is implemented by a single jle instruction, where as the latter requires an addition followed by a jl instruction.

http://en.wikibooks.org/wiki/X86_Assembly/Control_Flow#Jump_if_Less

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I wouldn't worry about this at all as far as performance goes. Using C as an example, on a simple test I ran with GCC 4.5.1 targeting x86 (with -O2), the (x <=y ) operation compiled to:

    // if (x <= y) {
    //     printf( "x <= y\n");
    // }
    //
    // `x` is [esp+28]
    // `y` is [esp+24]

    mov eax, DWORD PTR [esp+24]     // load `y` into eax 
    cmp DWORD PTR [esp+28], eax     // compare with `x`
    jle L5                          // if x < y, jump to the `true` block
L2:

    // ...

    ret

L5: // this prints "x <= y\n"
    mov DWORD PTR [esp], OFFSET FLAT:LC1
    call    _puts
    jmp L2      // jumps back to the code after the ` if statement

and the (x < y + 1) operation compiled to:

    // if (x < y +1) {
    //     printf( "x < y+1\n");
    // }
    //
    // `x` is [esp+28]
    // `y` is [esp+24]

    mov eax, DWORD PTR [esp+28]     // load x into eax
    cmp DWORD PTR [esp+24], eax     // compare with y
    jl  L3                          //  jump past the true block if (y < x)
    mov DWORD PTR [esp], OFFSET FLAT:LC2
    call    _puts
L3:

So you might have a difference of a jump around a jump or so, but you should really only be concerned about this kind of thing for the odd time where it really is a hot spot. Of course there may be differences between languages and what exactly happens might depend on the type of objects that are being compared. But I'd still not worry about this at all as far as performance is concerned (until it became a demonstrated performance issue - which I'll be surprised if it ever does for more than once or twice in my lifetime).

So, I think the only two reasons to worry about which test to use are:

• correctness - of course, this trumps any other consideration
• style/readabilty

While you might not think there's much to the style/readability consideration, I do worry about this a little. In my C and C++ code today, I'd favor using the < operator over <= because I think loops tend to terminate 'better' using a < than a <= test. So, for example:

• iterating over an array by index, you should typically use an index < number_of_elements test
• iterating over an array using pointers to elements should use a ptr < (array + number_of_elements) test

Actually even in C, I now tend to use a ptr != (array + number_of_elements) since I've gotten used to STL iterators where the < relation won work.

In fact, if I see a <= test in a for loop condition, I take a close look - often there's a bug lurking. I consider it an anti-pattern.

Noe I'll grant that a lot of this may not hold for other languages, but I be surprised if when I'm using another language that there's ever a performance issues I'll have to worry about because I chose to use < over <=.

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What data-type?

If y is INT_MAX, then the first expression is true no matter what x is (assuming x is the same or smaller type), while the second expression is always false.

If the answer doesn't need to be right, you can get it even faster.

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Have you considered that both those arguments are different? In case x and y are floating point numbers - they may not give the same result. That is the reason both comparison operators exists.

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Prefer the first one.

In some languages with dynamic types the running environment has to figure out what the type of y is and execute the appropriate + operator.

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Leaving this as vague as you have has caused this to be an unanswerable question. Performance cannot be evaluated unless you have software and hardware to measure - what language? what language implementation? what target CPU architecture? etc.

That being said, both <= and < are often identical performance-wise, because they are logically equivalent to > and >=, just with swapped destinations for the underlying goto's (branch instructions), or swapped logic for the underlying "true/false" evaluation.

If you're programming in C or C++, the compiler may be able to figure out what you're doing, and swap in the faster alternative, anyway.

Write code that is understandable, maintainable, correct, and performant, in that order. For performance, find tools to measure the performance of your whole program, and spend your time wisely. Optimize bottlenecks only until your program is fast enough. Spend the time you save by making better code, or making more cool features :)